1. Field of the Invention
The invention relates to an electric power steering system.
2. Description of the Related Art
In an electric power steering system (EPS) that uses a motor as a drive source, various compensation controls are executed in order to provide a good steering feel.
The states of a steering operation performed by a driver (steering states) are broadly grouped into the steering angle increasing state, the steering angle maintained state, and the steering angle decreasing state. In the steering angle increasing state, the steering angle is increased. In the steering angle maintained state, the steering angle is maintained. In the steering angle deceasing state, the steering angle is decreased. The steering feel required by the driver varies depending on the steering states. The driver tends to have an uncomfortable steering feel especially during the transition of the steering states.
To address the above-described problem, Japanese Patent Application Publication No. 2006-142932 (JP-A-2006-142932) describes a technology relating to an EPS. According to the technology described in JP-A-2006-142932, the value of steering torque, used as the basis for calculation of the base assist control amount, is corrected based on the steering state at a stage prior to the calculation of the base assist control amount (steering torque shift control). That is, correcting the steering torque at the stage prior to the calculation of the base assist control amount makes it possible to obtain, with a simple configuration, the assist characteristic that is required in each steering state although the required assist characteristic varies depending on the steering states. More specifically, in the steering angle maintained state and the steering angle decreasing state, the steering torque shift control is executed in such a manner that the absolute value of the base assist control amount increases. Thus, it is possible to reduce the effort of the driver required in the steering angle maintained state, and to reduce an uncomfortable feel that is given to the driver during the transition from the steering angle maintained state to the steering angle decreasing state. As a result, it is possible to provide a better steering feel.
In the steering torque shift control according to the technology described in JP-A-2006-142932, the compensation component in accordance with the steering state is calculated by multiplying the basic compensation component by the transition coefficient. The basic compensation component is used to change the steering torque to increase the assist force. The transition coefficient is continuously changed in response to the transition of the steering states indicated by the steering speed. Also, the compensation component is subjected to the abrupt change prevention process that is executed with the use of a low-pass filter. Thus, it is possible to suppress a sharp change in the assist force. As a result, it is possible to provide a good steering feel even during the transition of the steering states.
However, if improvement of the basic performance, for example, the response to a steering operation, is promoted, the rate of change in the transition coefficient becomes higher. As a result, a slight change in the steering speed may cause a great change in the output. Thus, the influence of noise may be more significant, and hunting may occur in the output. In addition, the transition coefficient differs in frequency band from the compensation component obtained by multiplying the basic compensation component by the transition coefficient as described above. Therefore, there is a possibility that the filter process executed on the compensation component will not provide effective measures.